Artificial respiration of patients is carried out according to the state of the art by means of respirators which send a breathing gas according to the positive pressure method to the patient. The breathing gas is either drawn in by a fan from the environment or it enters a humidifying device from a central gas supply system. Humidifying devices are used during the respiration of a patient to add water vapor to the breathing gas fed to the patient during each phase of inspiration in order to humidify the essentially dry breathing gas drawn in from the environment or introduced from the central gas supply system in a temperature-controlled manner or to an extent at or close to full moisture saturation.
Such a humidifying device is known from U.S. Pat. No. 6,272,933 B1. The humidifying device contains a refillable evaporating chamber, which is intended to receive water that is to be evaporated. The evaporating chamber is arranged in contact with a heater within a housing. The evaporating chamber contains a water feed means, into which water enters under the force of gravity from a water reservoir through an opening. Furthermore, breathing gas, which will be heated and humidified in the evaporating chamber, enters through another opening into the evaporating chamber. The water reserve in the evaporating chamber may be used up rapidly during the respiration of the patients, which often lasts for days, with a corresponding humidification of the breathing gas. A necessary refilling of the water in the evaporating chamber is carried out according to the state of the art continuously by means of a valve technique developed especially for this purpose, as it is described, for example, in DE 693 23 518 T2. However, automatic filling of the evaporating chamber with water is limited because of the limited volume of the water reservoir, as a result of which further humidification of the breathing gases cannot take place any longer when the water in the reservoir has been consumed.
A solution to this problem is described in U.S. Pat. No. 6,272,933 B1, according to which the data of a temperature sensor arranged on the breathing gas outlet channel of the evaporating chamber and of a measured signal of the breathing gas flow through the chambers are analyzed. An algorithm determines from this a state of the evaporating chamber without water. The drawback of this solution is, however, the fact that an alarm can only be triggered with a time delay, namely, after the analysis has been concluded, as a consequence of which the course of a therapy may be possibly affected.